Annually millions of cubic meters of wastewater, known as flowback and produced water (FPW), is generated by hydraulic fracturing for the extraction of oil and gas. FPW is typically saline, ranging from 10,000’s to over 300,000 ppm TDS, and it hosts a variety of organic compounds associated with the hydraulic fracturing fluids, the geologic formation and reaction byproducts generated by high heat in the subsurface and pressure generated during the fracturing process. The lack of publications on the risks associated with the hydraulic fracturing process or the composition and toxicology of FPW either as a whole or its individual components has generated significant public concern around fracturing wastewater generation and handling. To fill this gap, we investigated the composition of secondary precipitates in FPW from the Duvernay Formation in Alberta, Canada and their contribution to toxicity. The FPW secondary precipitates were characterized using electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction, and by chemical digestion. They were found to be primarily amorphous silica doped ferric iron oxides with trace strontium and barium sulfates, and were able to bind a range of organic compounds including PAHs. Toxicity analyses of zebrafish embryos with EROD and LC50 calculations showed approximately double the response when exposed to FPW samples with secondary precipitates as opposed to sediment free samples. When sampled at the oil and gas separator, the FPW was rich in dissolved Fe(II) 80-180 ppm, 32-78 °C and likely oversaturated with respect to Si at ambient surface temperatures. Therefore, we presume that the formation of the silica doped iron oxides happens as a function of both cooling of fresh FPW from the temperature of the geologic formation to ambient temperatures, and by the diffusion of oxygen once exposed to the atmosphere at the surface. Ultimately, we found that secondary precipitates may comprise a large fraction of the overall toxicity of FPW. The development of treatment technologies to handle the secondary precipitates may prove promising in reducing the overall environmental risk posed by FPW.

Characterization of secondary precipitates in flowback water from hydraulic fracturing and their contribution to zebrafish embryo toxicity

253rd National Meeting (2017)

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